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Stream Ecology: The Wild World Down Under

Imagine trying to live in a stream instead of on land. It would be like living in a constant hurricane. Not only would you have to extract oxygen from water, and reproduce in the maelstrom, you would have to fight for your life to stay put or find shelter, and, you would have to gather and eat food. Most of the food items available to you, rush by as they’re swept downstream.

Allochthonous Input: Outside Energy Sources
Food comes in many different forms for different creatures in the river, and the food web spreads well beyond the banks of the river. Allochthonous inputs are energy sources from outside the water system, like leaves, cones, branches, and logs that fall into a river.

When a branch first falls into the water as allochthonous input, it's called Course Particulate Organic Matter (or CPOM). Immediately, microbes like fungus and bacteria attack and decompose the branch, river turbulence thrashes it into pieces, and insects called “shredders” break it down further. As the organic material breaks down, it becomes Fine Particulate Organic Matter (or FPOM, also known as detritus) – a more palatable food for aquatic invertebrates like insects to chew and digest. Aquatic insects eat allochthonous inputs. Fish eat the aquatic insects. The cycle continues, but it does not stop there. Even smaller processes occur as this branch decomposes and feeds another community of organisms.

As river turbulence, microbial action, and the chewing and digestion of aquatic insects turn course matter into fine matter, Dissolved Organic Matter (or DOM, also known as Dissolved Organic Carbon) leaches into the stream.

This energy created from decomposing allochthonous input (i.e. - branches, leaves, and cones) provides nutrients to autochthonous organic matter downstream, like aquatic plants and algae, supports the growth of microorganisms, and closes the microbial loop of the river food cycle.

Autochthonous Input: Green Plants in the Stream
While allochthonous input enters the river from the surrounding forest, autochthonous input are energy sources native to the watery ecosystem. If you have ever slipped on a slick river rock, then you have experienced autochthonous input in the form of biofilm – an important part of the river food web.

Biofilm is a slimy substance that grows on rocks and pebbles on the riverbed and riverbanks. This gel-like substance is a mixture of sugars, enzymes, diatoms (attached and loose), bluegreen algae, bacteria, and protozoans. Absorbing Dissolved Organic Matter from the water and collecting Fine Particulate Organic Matter, biofilm feeds certain aquatic insects called “collectors” and “grazers” upon which many fish and predatory insects feed – returning nutrients to the river food web.

When a branch drops into Indian River, it decomposes from coarse to fine to dissolved organic matter, feeding increasingly smaller organisms as it breaks down. Inversely, those small organisms provide food for increasingly larger animals – even salmon, bears, and other keystone species.